Using PolyPen in Retopoflow 2.0
I was able to find the answer to my question by looking at a tutorial for the previous version of Retopoflow. 

You need to right click on bottom edge of the existing geometry where you want to place your quad. That edge will be the top of the new quad. Then hold down Control and left drag the mouse diagonally. You'll see a temporary triangle. Click on the empty corner at the bottom of your new quad where you will place a vert. Voilà. You can make tris this way as well.

Check out the tutorials for the old version and the current version to see this in action.

Enjoy. This addon is a blockbuster!

Sabia